Inheritance and polymorphism wouldn't work anyway because the method to call has already been established. I think that ysth's point is that the two forms are not 100% equivalent, because there's some means to tell them apart. This seems to privilege the $object->$method() notation over the other IMHO.

Whether this information will ever be used is something that only the particular implementation will be able to tell, but I'd inclined to think that in the vast majority of cases the two forms are interchangeable without any problem. But I'm eager to see comments contradicting this statement!